Hi Jeane, The census returns from the 19th century were either destroyed by the British government or lost in the fire at the PRO in the Four Courts at the start of the Irish Civil War in 1922. Official births, deaths and marriages date from 1864, so before that people rely on church records. As you do not have a parish, I'd suggest the best way forward is to post a message with a header such as 'Griffith's Look-up Please' and ask for readers to check out Brennan in Co. Laois. This will give some indication of where in the county the name occurred in the 1850's. Good luck!